feat: 首页和资产页面使用 CoinIcon 显示币种图标

✨ 修改内容：
- 首页 (home_page.dart): 持仓列表使用 CoinIcon 替换 CircleAvatar
- 资产持仓 (holdings_section.dart): 使用 CoinIcon 替换文字显示

🎨 效果：
- 首页：持仓卡片显示真实的币种图标
- 资产页面：持仓列表显示真实的币种图标
- 未找到图标时自动显示币种代码（兜底方案）

- ✅ Flutter Web 构建成功 (23.7s)

flutter_monisuo/build/web/main.dart.js_109.part.js

@@ -23,14 +23,14 @@ break A}if("input.selectAll"===e){y="\u0633\u0628 \u0645\u0646\u062a\u062e\u0628
 break A}if("keyboardToolbar.done"===e){y="\u06c1\u0648 \u06af\u06cc\u0627"
 break A}y=null
 break A}return y},
-br_(d){return new A.aVC(d)},
+br_(d){return new A.aVB(d)},
 a04:function a04(d,e){var _=this
 _.r=d
 _.z=_.w=$
 _.a=e
 _.e=_.b=$},
-aRw:function aRw(){},
-aVC:function aVC(d){this.a=d},
+aRv:function aRv(){},
+aVB:function aVB(d){this.a=d},
 acd:function acd(){}}
 B=c[0]
 A=a.updateHolder(c[79],A)
@@ -39,22 +39,22 @@ h(d,e){var y=this.r.a7(e)
 return y==null?this.a.a7(e):y},
 gb9(){var y=this,x=y.z
 if(x===$){if(y.w===$)y.w=y
-x=y.z=new A.aRw()}return x},
+x=y.z=new A.aRv()}return x},
 gbe(){return this.r}}
-A.aRw.prototype={
+A.aRv.prototype={
 gbj(){return"\u06a9\u0627\u0679\u06cc\u06ba"},
 gbi(){return"\u06a9\u0627\u067e\u06cc \u06a9\u0631\u06cc\u06ba"},
 gbk(){return"\u0686\u0633\u067e\u0627\u06ba \u06a9\u0631\u06cc\u06ba"},
 gbg(){return"\u0633\u0628 \u0645\u0646\u062a\u062e\u0628 \u06a9\u0631\u06cc\u06ba"}}
 A.acd.prototype={}
 var z=a.updateTypes([])
-A.aVC.prototype={
+A.aVB.prototype={
 $1(d){return A.bpG(this.a,d)},
 $S:2};(function inheritance(){var y=a.mixin,x=a.inherit
 x(A.acd,B.Z)
 x(A.a04,A.acd)
-x(A.aRw,B.b4)
-x(A.aVC,B.aW)
+x(A.aRv,B.b4)
+x(A.aVB,B.aW)
 y(A.acd,B.b_)})()
 B.b9(b.typeUniverse,JSON.parse('{"a04":{"Z":[]}}'))};
-(a=>{a["IAEHCpVS4dBpvHCrhal0jSFRHlQ="]=a.current})($__dart_deferred_initializers__);
+(a=>{a["vOebsZuT13NAdJXrPkLyUVsD39Y="]=a.current})($__dart_deferred_initializers__);